Week 8 Review Sheet answers

Week 8 Review Sheet answers - Final Review Sheet Week 8...

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Final Review Sheet – Week 8 Lecture 20 Main Points Cell-cell junctions use specific molecules to connect different cells. Two types of cell-cell junctions use modified cadherins to connect. Adherens junctions, or belt desmosomes, have cadherins in the cell membranes of opposing cells. On the intracellular side, these proteins are linked through linkers to actin. The actin forms a ring around the interior of the cell and can contract to change the shape of the cell. In desmosomes, there are intermediate filament proteins linked to the cadherins. These IF proteins traverse the cell’s interior, linking to desmosomes on either side of the cell. These IF proteins are very strong and stable and provide structural integrity to the epithelial layer. The junctions mentioned previously, that is desmosomes and adherens junctions share the features that they have cadherins in the membrane and specific cytoskeletal elements on the cytoplasmic side. Adherens junctions use actin and hemidesmosomes intermediate filaments. Tight junctions use unique proteins to seal cells together so that no molecules can pass between them. These junctions are important in epithelia where such seals are critical. For example, tight junctions make it such that nothing can pass between the epithelial cells lining the intestine, meaning that all material that is to be absorbed into the body must come through cells. The structure of tight junctions involves alignment of spherical junctional proteins into a bead-like arrangement, with strings of proteins running in parallel on each cell and with the individual proteins opposing each other attached. There are interlaced strings of these proteins, so that the area becomes repetitively sealed. A second role for tight junctions is to prevent migration of membrane proteins from one region of the cell to another. This allows for the polarity of cells, with different sides specialized to perform different tasks (e.g. the intestinal epithelial cell with different glucose transport molecules on different sides). Gap junctions are made of a ring of six protein subunits, connexons, forming an open hole on one membrane. A similar ring is on the adjacent cell. These holes align, allowing small molecules to pass directly from the cytoplasm of one cell to the other. Molecular weight cut off is about 1,000 Da, meaning that ions and regulatory molecules such as nucleotides can pass through. Plant cell walls have a structure that looks different but plays a similar role to the gap junction. These structures, plasmadesmata, are lined with plasma membrane that is contiguous from one cell to the next. In the center of the structure is a “desmotubule”, an extension of the ER. Using fluorescent tags, One can estimate the size of molecules that can pass through plasmadesmata. In the book are pictures that indicate that a protein can go through, but its mRNA (which is larger
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Week 8 Review Sheet answers - Final Review Sheet Week 8...

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